Variables

Pepper

Variable

• A variable – box – holds a certain type of value– value inside the box can change

• Example– A = 2B+1– Slope = change in y / change in x– monopoly square – holds different monopoly

pieces

Data types

• type: A category or set of data values.–Examples: integer, real number, string.

• Internally, the computer stores all data as 0s and 1s.

–examples: 42 101010"hi"

0110100001101001

Java's primitive types• primitive types: Java's built-in simple data types for

numbers, text characters, and logic.– Java has eight primitive types.– Types that are not primitive are called object types.

• Four primitive types we will use:

Name Description Examples– int integers (whole numbers) 42, -3, 0, 926394

– double real numbers 3.14, -0.25, 9.4e3

– char single text characters 'a', 'X', '?', '\n'

– boolean logical values true, false

Types

•

type kind memory range

byte integer 1 byte -128 to 127

short integer 2 bytes -32768 to 32767

int integer 4 bytes -2147483648 to 2147483647

long integer 8 bytes -9223372036854775808 to-9223372036854775807    

float floating point

4 bytes ±3.40282347 x 1038 to±3.40282347 x 10-45  

double floating point

8 bytes ±1.76769313486231570 x 10308 to

±4.94065645841246544 x 10-

324     

char single character

2 bytes all Unicode characters

boolean true or false 1 bit  

Create a variable• create a variable

– In Java: <type> <variable name> ;  example: int myIntVar;

– declare with correct type: int, double, char, boolean, (not capitalized)

– declare Object variables with types such as : String, Scanner, Random, (capitalized)  

• You try: – Create a program that creates the following variables:

• int myQuantity• double myDollars• boolean answer• char oneLetter• String myName

Change a variable• change variable contents:

– In Java: <variable name> = <some expression>; example:  myIntVar = 5 ;.

– Equal sign means set equal to – Left side gets put into right side

• You try – set those variables = to something• myQuantity = 1• myDollars = 3.3• answer = true• oneLetter = ‘a’• myName = “pepper”

• Now, print those variables with: – System.out.println(myQuantity + myDollars + answer +

oneLetter + myName)• What happens when you put single or double quotes

where there were none, or take away the quotes?

Use variables in a real program

• On Paper: Average 2 + 4 + 6

• Pay attention to your steps

Writing the Average program

Specification – write a program which can find the average of three numbers.

Let’s list the steps that our program must perform to do this:• Add up these values

• Divide the sum by the number of values

• Print the result

Each of these steps will be a different statement.

Flow chart

• Done in Gliffy

• https://www.gliffy.com/

• Show how

Writing the Average program

• Add up these values• Divide the sum by the number of values• Print the result

sum = 2 + 4 + 6;

sum = 2 + 4 + 6; an assignment statement

Assignment Statements

• Assignment statements take the form:variable = expression

Memory location where the value is stored Combination of constants

and variables

Expressions

• Expressions combine values using one of several operations.

• The operations being used is indicated by the operator:

+ Addition- Subtraction* Multiplication/ DivisionExamples:

2 + 53 4 * valuex / y

Writing the Average program 1

• sum = 2 + 4 + 6;• Divide the sum by the

number of values• Print the result

average = sum / 3;

Names that describe whatthe values represent

Writing the Average program 2

• sum = 2 + 4 + 6• average = sum / 3;• Print the result

System.out.println(″The average is ″ + average); The output method variable

name

Writing the Average program 3

public static void main(String[] args) {

-------------------- sum = 2 + 4 + 6; average = sum / 3; System.out.println("The average is "

+ average); }

We still need to add a declare our variables. This tells the computer what they are.

Writing the Average program 4

public class Average3 { public static void main(String[] args) { int sum, average; sum = 2 + 4 + 6; average = sum / 3; System.out.println("The average is " +

average); }}

Tells the computer that sum and average are integers

Writing the Average program 5

public class Average3a {

public static void main(String[] args) {

int sum;

int average;

sum = 2 + 4 + 6;

average = sum / 3;

System.out.println("The average is " + average);

}

}

We could also write this as two separate declarations.

You try

• Add 1 to the average and then print it again.

average = average + 1

Variables and Identifiers

• Variables have names – we call these names identifiers.

• Identifiers identify various elements of a program (so far the only such element are the variables.

• Some identifiers are standard (such as System)

Identifier Rules

• An identifier must begin with a letter or an underscore _

• Java is case sensitive upper case (capital) or lower case letters are considered different characters. Average, average and AVERAGE are three different identifiers.

• Numbers can also appear after the first character.

• Identifiers can be as long as you want but names that are too long usually are too cumbersome.

• Identifiers cannot be reserved words (special words like int, main, etc.)

Spelling Conventions

• Name constants

• Variables start lower case

• Classes uppercase

• Word boundaries upper case (numberOfPods)

Some Illegal Identifiers

timeAndAHalf& is not allowed

time&ahalf

fourTimesFive* is not allowedfour*five

times2 or twoTimes

Cannot begin with a number

2times

myAgeBlanks are not allowed

my age

Suggested IdentifierReasonIllegal Identifier

Assignment

int number1 = 33;

double number2;

number2 = number1;

byteshortintlongfloatdoublechar

Dividing

• int / int int (even if you assign it to a double)

• float / int float

• int / float float

Solution: Cast it

ans = n / (double) m

Math Operators & PEMDAS

• + add

• - subtract

• * multiply

• - division

• % remainder

Example: base + (rate * hours)

Fancy Math

variable = variable op (expression)count = count + 1 count = count + (6 / 2 * a + 3)variable op = expressioncount += 1 count += (6 / 2 * a + 3)

Example:int count = 1;count += 2;The value of count is now 3

More Fancy Math

• Increment ++

• Decrment –

• ++n adds 1 before executing

• n++ adds 1 after executing

Example:

Constants

Constant doesn’t change

Why use a variable if

massive changes later

show meaning

avoid Hard coding

public static final int MAX_PEOPLE = 20;

Capitalize by convention only

Summary

• A variable holds one value

• Variables have certain types

• Literals of different types are entered differently (i.e. quotes for String)

• The system keeps variable values until you change them.

• Constants are a special kind of variable – no changing